In recent years, non-linear optical materials have been attracting increasing attention. Under favorable conditions (e.g., high intensity, phase matching) they convert light of a given wavelength into light of a different wavelength. Non-linear optical devices have utilized the non-zero components of the second order polarizability tensor to achieve second harmonic generation, parametric amplification, the addition and subtraction of frequencies, modulation and the like of coherent electromagnetic reaction.
Such materials have been generally known as non-linear optical materials, and described in detail in references as mentioned below: "Nonlinear Optical Properties of Organic and Polymeric Material", ACS Symposium Series 233, edited by David J. Williams (published by American Chemical Society, 1983); "Yuki Hisenkei Kogaku Zairyo; Organic Non-linear Optical Material", supervised by Masao Kato, Hachiro Nakanishi (CMC, published in 1985); "Nonlinear Optical Properties of Organic Molecules and Crystals", vol. 1 and vol. 2, edited by D. S. Chemla and J. Zyss (published by Academic Press in 1987).
As one of the popular uses of non-linear optical materials, there is the wavelength converting device using second harmonic generation (SHG) or frequency doubling based on the non-linear effect and the addition of frequencies and the subtraction of frequencies.
Briefly, electromagnetic waves propagating in a crystal having non-linear optical properties induce polarization waves with frequencies which are the sum and the difference of the frequencies of the exciting waves. These polarization waves can radiate electromagnetic waves having the frequencies of the polarization waves. The energy transferred to a radiated electromagnetic wave from a polarization wave depends on the magnitude of the component of the second order polarizability tensor involved, since this tensor element determines the amplitude of the polarization wave and also depends on the distance over which the polarization wave and the radiated electromagnetic wave can remain sufficiently in phase, called the coherence length. Phase matching occurs when the waves are completely in phase.
Generally phase matching is of two types:
(i) Type I, wherein the two incident waves have the same polarizations; and
(ii) Type II, wherein the two incident waves have orthogonal polarization.
Phase matching can be achieved by "tuning" the crystal in various ways such as by rotation of the crystal to vary the refractive indices, by varying the temperature, by application of an electric field, or by compositional variation.
Those non-linear optical materials which have been used up to date are inorganic perovskites as represented by lithium niobate. Recently, however, .pi.-electron conjugated system organic compounds having an electron donating group and an electron attracting group have been known to have various performances as the non-linear optical materials surpassing greatly those of the inorganic materials mentioned above.
For formation of non-linear optical materials having higher performances, compounds having higher non-linear sensitivities under molecular state are required to be aligned so as to give rise to no inverse symmetry. For atainment of high non-linear sensitivity, compounds with a long .pi.-electron conjugated chain have been known to be useful, and described variously in the above-mentioned literatures. However, in those compounds, the absorption maximum wavelength will become longer as is self-evident, bringing about, for example, lowering in transmittance of blue light, which is an impediment against generation of blue light as the second harmonic. This also occurs in p-nitroaniline derivatives, and the great effect of the wavelength on efficiency of the second harmonic generation is apparent from Alain Azema et al, Proceedings of SPIE, vol. 400, New Optical Materials, 1983, p. 186, FIG. 4.
Therefore, it has been desired to have a nonlinear optical material with higher transmittance to blue light. In the prior art, investigations have been made involving replacement of the carbon atoms on the benzene nucleus of nitroaniline with nitrogen atoms, but no satisfactory result has been obtained.
Also, more excellent methods have been disclosed in JP-A-62-210430 (the term JP-A as used herein means an "unexamined published Japanese patent application") and JP-A-62-210432.
However, as described previously, to be useful as the secondary non-linear optical material, not only must the performance in the molecular state be sufficient, but it is essentially required that the molecular alignment under gathered state (i.e., incrystalline form) should have no inverse symmetry. It is extremely difficult to predict molecular alignment, and the probability for existence of satisfactory properties for any given organic compound is not high.